In a nuclear reactor pressure vessel, reactor internals assemblies, normally in the form of both upper and lower internals, are used to support and orient reactor core fuel assemblies, utilized to generate heat for the production of steam and electricity, provide a passageway for reactor coolant, typically borated water, and support in-core instrumentation. One such structure is commonly referred to as the reactor lower internals and includes upper and lower core barrels, thermal shield, lower core plate, and various other structures and components to effectively provide for the above-stated objectives.
The lower internals are supported both from the top and bottom by the reactor vessel. At its top portion, the lower internals assembly includes an outwardly directed radial flange which engages an inwardly directed internals support ledge on the interior of the reactor vessel. The internals support ledge is part of the reactor vessel flange upon the top surface of which is sealingly engaged the reactor vessel closure head by a plurality of stud bolts, in a manner well known in the art. The lower end of the lower internals assembly is restrained from transverse movement by a radial support system attached to the vessel wall. This radial support system is achieved by a key and keyway arrangement. Typically, at a plurality of equally spaced points around its circumference, blocks are welded to the inside diameter of the reactor vessel, each of which has a keyway geometry. Opposite each of these blocks is a key which is attached to the lower end of the lower internals assembly. When the lower internals are installed within the reactor vessel, the keys engage the keyways in the axial direction to provide support at the bottom of the lower internals.
Occasionally the lower internals are removed from within the reactor vessel for inspection and/or maintenance thereof. Since an operating nuclear reactor generates an irradiated environment, which is safely protected within a containment building, borated water or other shielding medium is maintained over the irradiated components normally maintained within the interior of the reactor pressure vessel. Typically, when the lower internals are removed, the structure is transported to a location within the containment building and placed on a storage stand. Although the containment building defines a large interior space for the storage of reactor components during maintenance operations, a typical reactor lower internals structure is a relatively large component and may not be capable of being completely submerged when so stored. Because of this, large auxiliary shielding structures must be provided within the containment structure to safely protect maintenance personnel, and to lower overall man-rem exposure levels. Additionally, an improved reactor vessel lower internals storage area arrangement is disclosed in patent application Ser. No. 213,209, filed on June 29, 1988, and assigned to the present assignee. This arrangement comprises support ledges and columns for the upper internals and lower internals structures integrally formed within the floor and walls of the refueling cavity at the time of initial plant construction.
Whereas these structures can adequately protect maintenance personnel, their construction can be time consuming and costly for existing plants. Not only must a separate structure be provided for and housed within the containment building, it must be put in place prior to the commencement of the particular maintenance procedure. This adds to the time required for such procedures, and subtracts from the time during which the nuclear reactor power plant can be operating and generating electricity. It is highly desirable then, that a less expensive means be available for adequately shielding the nuclear reactor's lower internal assembly when it is removed therefrom.